The present invention generally relates to fabrication of semiconductor devices and more particularly to a dry etching apparatus used for fabricating semiconductor devices.
The technique of dry etching, particularly the plasma etching, is an important process in the fabrication of semiconductor devices. A typical plasma etching apparatus has a pair of parallel electrode plates provided in a reaction chamber to oppose with each other, and carries out etching of an object by an etching reaction that is enhanced by a plasma formed between the electrodes. For this purpose, a high frequency electric field is established across the opposing electrodes, and the object is placed on one of the electrodes to face the other electrode.
The plasma etching apparatuses used in these days generally have a device for monitoring the thickness of the layer that is subjected to the etching such that a precise control of etching can be achieved. A typical example of such a device employs a laser beam for this purpose. More specifically, an object such as a semiconductor substrate is placed on one of the electrodes to face the other, opposing electrode, and a laser beam is irradiated on the substrate through an aperture provided on the opposing electrode. Further, a detection system is provided outside the reaction chamber for detecting a reflected laser beam produced upon reflection of the incident laser beam at the substrate. For this purpose, an optical device is provided so to intercept the reflected laser beam that exits from the aperture in the reversed direction to the incident laser beam.
Such a plasma etching apparatus generally has a transparent cover of quartz, for example, on the opposing electrode on which the aperture is provided to protect the electrode from unwanted etching. Thereby, unwanted contamination of the substrate by the deposition of metal such as aluminum that forms the electrode is avoided. The transparent cover further permits the passage of the incident and reflected laser beams, and thus, an exact control of the etching can be achieved.
In such a conventional plasma etching apparatus, however, there arises a problem in that various materials that are formed as a result of etching of the substrate are deposited on the inner wall of the reaction chamber, including the transparent cover. When such a deposition of materials occurs, it is necessary to break the vacuum of the reaction chamber and clean up the reaction chamber regularly. Such a cleaning of the transparent cover has to be made with about every 50 hours.
It should be noted that such deposition of materials occurs also on the quartz cover that covers the aperture of the opposing electrode. Thereby, the transmittance of the laser beam employed for the detection of the progress of the etching is gradually decreased. Because of this, the reliable detection of the end of etching is no longer possible when the apparatus is operated, for example, about 10 hours. This duration is significantly shorter than the interval between the cleaning operations of the reaction chamber. Thus, in the actual operation of the apparatus, one has to break the vacuum of the reaction chamber about every 10 hours, instead of every 50 hours, for cleaning the transparent cover. Each time such a break of vacuum is made, a preliminary running procedure is needed at the time of restarting the operation, to eliminate any moisture or gas that has been adsorbed on the wall of the reaction chamber. Thus, the problem of cleaning up of the transparent cover provides a serious problem in the efficiency of the etching procedure particularly when used in a mass-produce line.